This invention relates to a cast alloy having combined corrosion and abrasion resistance.
Applicant is aware of the following U.S. patents, the disclosures of which are incorporated by reference herein: U.S. Pat. Nos. 2,212,496; 2,311,878; 2,323,120; 3,165,400; 3,250,612; 3,876,475 and 3,941,589, and United Kingdom No. 362,975 of 1931.
Equipment used in corrosive environments is typically constructed of stainless steel or other high alloy materials. These alloys provide excellent service in clear fluids. However, when subjected to a corrosive slurry, fluid containing abrasive solids, under moderate to high velocity, these materials perform poorly due to poor abrasion resistance.
Equipment used in abrasive slurry environments is typically constructed of wear resistant irons. Wear resistant irons provide excellent service in neutral slurries. However, if the slurry becomes mildly acidic, these materials fail in short order due to inadequate corrosion resistance.
The alloy of this invention provides superior combined corrosion and abrasion resistance for handling acidic slurries.
An application requiring such a material is the production of wet process prosphoric acid. The initial step in the process is the reaction of raw phosphate ore with concentrated sulphuric acid. Products of the reaction are phosphoric acid and calcium sulphate, along with both chemical and solid impurities. A typical product slurry analysis is 42% phosphoric acid, up to 1% chlorine and fluorine impurities, approximately 2.5% sulphuric acid, and 30 to 40% solids. The solids are mostly calcium sulphate and siliceous gangue (which is highly abrasive). The operating temperature for raw acid formation, and the slurry temperature, is usually above 50.degree. C., typically 80.degree. C. The alloy of the invention can be expected to offer significantly improved life compared to either stainless steels or wear resistant irons for fluid handling equipment and filtration equipment in this environment.
The advantages of applicant's invention are achieved by a cast, high chromium, ferritic, white iron alloy possessing combined corrosion and abrasion resistance in both the as-cast and age hardened condition. Typically the alloy contains from between about 0.75% to 1.5% carbon, up to about 0.85 silicon, between about 2.0% to 2.5% manganese, between about 2.0% to 3.0% molybdenum, between about 1.0% to 2.0% copper, between about 0.5% to 1.0% tungsten, between about 24% to 30% chromium and the balance being iron along with normal residual elements. Preferably the alloy contains between about 0.9 to 1.2% carbon, between about 26 to 28% chromium, and between about 0.4 to 0.75% silicon. The silicon content should be kept as low as possible, without reducing the castability of the alloy. Silicon adds fluidity to the alloy melt. However, silicon can reduce the corrosion resistance of the alloy in acidic media, particularly in media contaning halide ions. It is preferred that the silicon level be as low as possible while maintaining good castability in the alloy melt.
The combination of the alloying elements in the specified proportions yields a material having an as-cast microstructure of a high chromium ferritic matrix with approximately 30% of the alloy being a discontinuous complex phase. The discontinuous phase contains high alloy chromium, molybdenum and tungsten carbides which provide extreme hardness and abrasion resistance to the alloy. The abrasion resistance can be further enhanced, with little or no loss in corrosion resistance, by a low temperature age hardening heat treatment. The alloy in either the as-cast or age-hardened condition possesses excellent combined corrosion and abrasion resistance. The alloy is readily castable by standard foundry practice, and has adequate strength and ductility suitable for mechanical rotating equipment.
It is thus an object of applicant's invention to provide an alloy for use in acid slurries.
It is an object of applicant's invention to provide an alloy which is resistant to the environments common in the wet process production of phosphoric acid.
It is an object of applicant's invention to provide an alloy which is resistant to abrasive conditions as found in hot slurries.
It is an object of applicant's invention to provide an alloy which has combined abrasion and corrosion resistance.
It is a further object of applicant's invention to provide a white iron alloy which has mixed abrasion and corrosion resistance.
It is an object of applicant's invention to produce a white iron alloy having a ferritic matrix.
It is a further object of applicant's invention to provide a white iron alloy having a dispersed phase in a ferritic matrix, the dispersed phase containing carbides of chromium, tungsten and molybdenum and producing an alloy having high resistance to combined corrosive and abrasive conditions.
It is a further object of applicant's invention to provide a white iron alloy having corrosion resistance and abrasion resistance which is castable and hardenable.